Sheet metal punches have commonly been fabricated from a single piece of suitable steel to define an elongated body or punch mounting portion and a reduced diameter shank or piercing end portion, which portions are joined by an integral reenforcing fillet. This basic construction suffers from the disadvantage that the whole of the punch must be discarded in the event that there is a structural failure of the shank end portion or it becomes excessively worn after continued sharpening. Structural failure normally involves breaking of the shank end portion at approximately its junction with the reenforcing fillet, due to stresses created during the actual punching operation.
Over the years, many improvements have been made in one piece steel punches with a view to reducing the likelihood of structural failure. As by way of example, U.S. Pat. No. 2,431,567 discloses the utilization of a collar surrounding the junction between the shank end portion and fillet in order to provide a vibration absorbing support therefor.
Another approach, suggested for instance by U.S. Pat. Nos. 125,302; 770,238; 2,287,882; 2,323,756 and 2,808,108, has been to form a multi-part punch in order to permit replacement of a worn or broken shank end portion without the necessity of discarding the otherwise serviceable punch body portion. Punch constructions of this type have the added cost and structural design advantages of permitting the shank end and body portions of a punch to be formed from different types of steel, as for instance relatively high cost, high speed steel and a relatively low cost tool steel, respectively. However, the inherent drawback with such punch constructions is the inability to assemble the several parts in a manner providing for proper alignment of the parts and distribution of stresses encountered during the punching operation. As a result, the thickness of the material which may be punched without breakage of the replaceable shank end portion is limited to a thickness below that which may be successfully punched with a comparable one-piece punch.
In more recent years, it has been proposed to bond a "button" of tungsten carbide or other very hard, abrasive resistant material onto the tip end of the shank end portion of an otherwise integrally formed steel punch with a view to greatly increasing punch wear life, while maintaining to some extent the cost advantage of multi-part punch constructions. Again, however, certain drawbacks have been encountered. In this respect, very hard carbides, which have the best wear capabilities, present a serious problem with respect to the bonding of the "button" to the small tip end of shank end portion. For softer carbides, it is possible to achieve a reasonably adequate bond by procedures outlined in U.S. Pat. No. 3,279,049, but not without adversely influencing the quality of the steel punch body portion. In either case, there may be encountered fabricating problems in obtaining proper alignment of the "button" relative to the shank end portion in order to avoid the necessity of subsequent machine trimming operation.
Further, there have been prior attempts to form carbide punches by press fitting and/or brazing a generally cylindrical carbide punch element within a bore opening formed in a tool steel body, as disclosed for instance by U.S. Pat. No. 3,807,804. However, these prior attempts have involved high cost fabricating procedures and/or have resulted in punches having short useful life.